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Failure to obturate?

Discussion in 'Handguns: Revolvers' started by Owen Sparks, Apr 26, 2009.

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  1. Owen Sparks

    Owen Sparks member

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    I have noticed that mild loads in revolvers tend to have a black sooty smudge down one side of the empty brass case. Is this because the case is not obturating (expanding under pressure)? Would increasing the load stop this messy blowback?
     
  2. ArchAngelCD

    ArchAngelCD Member

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    If you're loading very hard bullets you will need higher velocities to prevent that "sooting". You are probably getting some leading too. If you don't want to increase the velocities you can change your bullet. Shooting a softer bullet will give you the same results as increasing the velocity on the harder bullet. (a hardness of 10-12)
     
  3. Steve C

    Steve C Member

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    Short answer is YES. Generally it is best when developing loads to start at 10% below maximum or at the listed start load, which is usually the same 10% but the maths done for you. A start load should produce enough jpressure to seal the case in the chamber. If you are loading a "special" light load for a particular purpose like a light load for new or recoil sensitive shooters, you'll just have to put up with the fouling on the case.

    ArchAngelCD brought up another issure regarding lead bullet base obturation and the need to match pressure with lead hardness to avoid leading. Hard bullets actually lead less (or not at all) with heavy loads while a soft bullet works well with lower pressure loads.

    If you pour your own you can size them properly to avoid leading but with commercial cast you rely on the base of the bullet obturating enough to fill the barrel which requires an appropriate level of pressure for the bullets hardness.
     
  4. Stainz

    Stainz Member

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    If the caliber is .45 Colt, you may have to exceed the SAAMI ratings for that to happen... unless you are shooting a custom or FA revolver with proper chamber IDs. Everyone else still uses the so-called 'black powder' ID dimension, which, being larger, still allows for black powder fouling. Of course, try to find bp loads in .45 Colt. Then you'll note that they are hotter - and a lot nastier - than the typical smokeless propellant .45 Colt load - especially 'cowboy' loads'. You'll note that smoke trail is usually on the vertically top side of the case.

    You can also experience it with larger chamber IDs from just careless, in my opinion, manufacturing. My favorite example were my Ruger .32s - an SSM and a 4" SP-101. Both allowed case swelling to .337+", while sizers and commercial ammo was .334" or smaller. So, it's not always lite loads.

    Stainz
     
  5. Glockman17366

    Glockman17366 Member

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    WOW!!!!!!!
    I learned a new word today! Obturate!
     
  6. harmon rabb

    harmon rabb Member

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    me too :cool:
     
  7. The Lone Haranguer

    The Lone Haranguer Member

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    225px-Cool_Hand_Luke_Martin.jpg

    Whut we got heah is ... failiah to obturate!
     
  8. Beagle-zebub

    Beagle-zebub Member

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    I'm glad I'm not the only one who thought what Lone Haranguer thought.
     
  9. logical

    logical Member

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    I think for cases, you should stick with "expand". Obturate has it's roots in "obstructing or blocking" and refers to a soft bullet filling a barrel because of the pressure squishing it from behind or from within the hollow cavity. cases get blown straight out...thats just expansion.
     
  10. Walkalong

    Walkalong Moderator

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    Yep. Cases expand to seal the chamber and stop gas blowby/sooting in that direction. Bullets obturate to fill and seal the throats/bore to stop gas cutting in that direction.

    Increasing pressure in the case will. More of the same powder, or use a faster powder that builds pressure more quickly. The faster powders are real handy for "light loads" in revolvers.

    ArchAngelCD was asleep at 12:12 this AM. He knows the answer. :D
     
  11. SaxonPig

    SaxonPig Member

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    Yes, low chamber pressure can result in sooty cases as they fail to expand and seal the chamber from blow-back.

    At this point I feel the need to point out that I have noted this is a common condition with factory +P 38 Specials meaning the chamber pressure is too low to seal the chamber. So much for +P being so hot.
     
  12. Matt-J2

    Matt-J2 Member

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    I don't think I can stuff any more BP into the cases! :p
     
  13. ArmedBear

    ArmedBear Member

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    I've generally seen this when I'm loading light (non-magnum or "cowboy") loads in magnum cases.

    .38 Special cases are built to obturate at a lower pressure than .357 Magnum cases, for example.

    A .38 Special loaded to 800 fps will obturate fine. Put a similar load in a .357 Magnum case and you'll see a lot of blow-by.

    .44 Magnum case walls can be so thinck that I have to wrestle with my press when resizing the brass. They REALLY resist obturation at low pressures. The fired cases generally fall right out of the revolver, too, no ejection required.
     
  14. Walkalong

    Walkalong Moderator

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    Brass expands, brass does not obturate. ;)

    Don't even........ just look up the definition of obturate. :neener:
     
  15. ArchAngelCD

    ArchAngelCD Member

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    Yeah, staying up all night doesn't mean I'm actually awake although I'm usually alright until after 4:00 AM!! LOL
     
  16. ArmedBear

    ArmedBear Member

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    Well, the brass doesn't expand, either. The actual brass does not change in volume at any point.

    The case, made of brass, really inflates and doesn't actually expand. It doesn't get bigger; the case wall stretches and gets thinner, while the diameter of the cylinder it forms gets larger.

    I'm not defending my misuse of "obturate". It really doesn't matter.

    The point is, heavier magnum cases take more pressure to stretch them out to fit the chamber tightly, than thinner-wall cases like the .38 Special.
     
  17. PRE 64 JOE

    PRE 64 JOE Member

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    obturate, I think my brain hurts cause I just figured out what y'all means.
     
  18. woad_yurt

    woad_yurt Member

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    Walkalong:
    The OP does have obdurate brass. There's no denying it.
     
  19. Noxx

    Noxx Member

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    I think it's more a function of +P cases being stronger. Obviousl the ballistics don't lie, 38+P is higher velocity.

    Anyhoot, I noticed the same thing when I started reloading, my first loads were very cautious, and I would tend to get a lot of case staining. After coming up to "factory" type loads in pressure and velocity, most of the problem went away, except with certain powders.
     
  20. ArmedBear

    ArmedBear Member

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    ...and when you worked up to just pouring in as much 296 as would fit in the case, and stuffing a bullet on top of it with a massive crimp to keep it in, your cases came out of the guns looking like they just got tumbled?:D

    What's a few blown primers and cracked cases when you can avoid cleaning them?:p

    (DISCLAIMER: THIS IS A JOKE. IT COULD BLOW UP YOUR GUN. DO NOT TRY THIS AT HOME!)
     
  21. Walkalong

    Walkalong Moderator

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    I'll buy that, along with strecthing as it "inflates" :)

    Absolutely.
     
  22. Noxx

    Noxx Member

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    296? Are you made of money man>?! I use Titegroup, it doubles as a desert topping.
     
  23. logical

    logical Member

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    You are being obtuse. Using the term "expand" to describe a brass case swelling is a perfectly correct use of the word. Nothing in the definition of expand says it changes in actual volume (I think you mean mass anyhow). A bird expands it's wings, a person expands their lungs, a ballon expands whem blown up.....

    Words matter.
     
  24. logical

    logical Member

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    double post
     
    Last edited: Apr 27, 2009
  25. Vern Humphrey

    Vern Humphrey Member

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    The proper terminology is "rearward obturation" which occurs when the brass expands and fills the chamber, and "forward obturaton" which occurs when the bullet fills the bore.

    If you have sooty brass, you're not getting rearward obturation.
     
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